src: don't use locale-sensitive strcasecmp()

strcasecmp() is affected by the current locale as configured through
e.g. the LC_ALL environment variable and the setlocale() libc function.

It can result in unpredictable results across systems so replace it with
a function that isn't susceptible to that.

PR-URL: https://github.com/nodejs/node/pull/6582


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>